Foamed resinous structures are useful in a wide variety of applications such as thermal insulation, in cushions, as packaging, and as adsorbents. Extruded foams are generally made by melting a polymer together with any desired additives to create a polymer melt. A blowing agent is mixed with the polymer melt at an appropriate temperature and pressure to produce a foamable gel mixture. The foamable gel mixture is then cooled and extruded into a zone of reduced pressure, which results in a foaming of the gel and the formation of the desired extruded foam product.
Traditional blowing agents used for extruded foam products include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). One of the advantages of both CFC and HCFC blowing agents is their high solubility in a polymer melt during the manufacturing process. Higher blowing agent solubility promotes a reduction in viscosity when the blowing agent is mixed with the polymer melt. In turn, lower viscosity leads to lower energy requirements for mixing. On the other hand, a major disadvantage to these traditional blowing agents is that an increasing number of governments worldwide have mandated the elimination of CFC and HCFC blowing agents due to growing environmental concerns. CFCs, and many other halocarbons, have come to be recognized as serious global environmental threats due to their ability to cause stratospheric ozone depletion and global warming. The ozone depletion and global warming impact of chemicals such as CFCs and HCFCs are measured by the ozone depletion potential (ODP) and global warming potential (GWP) respectively.
In view of the mandatory phase out of blowing agents with a high ODP and a high GWP, there has been a movement to replace the conventional blowing agents in favor of more environmentally friendly blowing agents, such as hydrofluorocarbons (HFCs) and CO2 in insulating foam applications. Although HCFCs provide a superior thermal barrier compared to CO2, the chlorine present in the HCFCs still possesses a small ozone depletion potential. Additionally, over time, the chlorofluorocarbon gas phase in the foam is released into the atmosphere, thereby reducing the insulative value of the foam and potentially contributing to the global warming potential. Further, each of these non-conventional blowing agents leads to a different cell size and morphology depending on the particular blowing agent chosen. Unfortunately, the cell sizes of the foam produced by these generally environmentally friendly blowing agents are too small to provide an acceptable insulative value to the foamed product.
Previously, there have been attempts in the art to utilize more environmentally friendly blowing agents or to modify the conventional HCFC blowing agents to reduce the ODP and GWP of the foam produced while maintaining the thermal insulation value of the foam. Some examples of these processes/compounds are described below.
U.S. Pat. No. 5,489,407 to Suh et al. describes a process for making a closed-cell, alkenyl aromatic polymer foam that has an enlarged cell size. The polymer is preferably polystyrene. For environmental reasons, the blowing agent is preferably an inorganic blowing agents such as CO2, nitrogen, argon, water, helium, or air. The composition used to form the foam includes a substantially non-waxy cell size enlarging agent. The cell size enlarging agent has diverse chemical structures, and includes compounds such as polyethylene glycol and polypropylene glycol with a molecular weight of 1200 or more, salts of n-tallow β-amino dipropionate, amine oxides, imidaxoline, fatty acid alkanolamides of C12 to C18, n-alkyl trimethyl ammonium chloride, ethoxylated linear alcohols, dioctyl ester sodium sulfoccunic acid, polyoxyethylene sorbitan monopalmitate, diglycol laurate, fluoro-chemical surfactants, coco betaine, aqueous emulsions and fluids of silicone compounds (e.g., dimethyl polysiloxane). The composition may optionally contain a nucleating agent (e.g., inorganic substances such as talc, clay, and/or calcium carbonate) to control the size of the foam cells.
U.S. Pat. No. 5,912,279 to Hammel et al. discloses a closed cell foam and a foaming agent that utilizes a hydrogen-containing halo-carbon blowing agent (e.g., HCFC-22) in combination with a hydrogen bond forming blocking agent (e.g., organic ether, ester, or ketone). The presence of the blocking agent reduces the escape of the blowing agent and entry of air into the foam to maintain a low thermal conductivity over a longer period of time. It is asserted that the blocking agent is capable of creating hydrogen bonds with the blowing agent, which dramatically reduces the permeation rate of the halo-carbon blowing agent out of the insulation foam (thereby reducing the global warming potential).
Hammel et al. also disclose a closed cell resinous foam that is formed utilizing an environmentally friendly and low permeable polyfluorocarbon blowing agent. The blowing agent includes more than about 70 weight percent of 1,1,2,2-tetrafluoroethane (HFC-134). The remaining 30 weight percent is formed of blowing agents that are devoid of halogen substituents other than fluorine. The foam body may be formed of any thermoplastic resin (e.g., polystyrene, polyethylene, or polypropylene). The blowing agents typically contain 1 to 2 carbon atoms and are substantially non-flammable, have zero ozone depletion potential, and a low halocarbon global warming potential.
U.S. Pat. No. 6,787,580 to Chonde et al. discloses a multi-modal thermoplastic and thermally insulating polymer foam that has a distribution of large and small cells in a substantial absence of water by using a blowing agent stabilizer. A preferred blowing agent is selected from non-ozone depleting blowing agents such as CO2, hydrocarbons, and hydrofluorocarbons, but not water. The blowing agent stabilizer creates domains of concentrated blowing agent that produce large cells within the foam. Blowing agent molecules that are not associated with the blowing agent stabilizer produce small cells. Suitable blowing agent stabilizers include polyethylene glycol (PEG) and PEG ethers, polyethylene oxide grafted polystyrene/maleic anhydride random copolymers, and ethylene glycol grafted polyurethane random copolymers. The foams may be in the form of planks or sheets, and are particularly useful as thermally insulating articles because they assertedly have high insulating values.
Despite these previous attempts to reduce the ODP and GWP, there remains a need in the art to achieve an extruded polymer foam that has an increased cell size when non-HCFC blowing agents are used, that maintains the positive physical properties of conventional extruded polystyrene foams, and that meets the stringent requirements for a reduction in the global warming potential and ozone depletion potential.